1. Field of the Invention
The present invention relates generally to nondestructive testing (NDT) of ferromagnetic ropes, cables, strands and prestressed tendons (in concrete) for flaws and fractures. The present invention relates more specifically to the nondestructive evaluation of ferromagnetic ropes, cables, strands, and prestressed tendons for flaws and fractures using magnetostrictively induced acoustic/ultrasonic waves, and the passive monitoring of crack growth and fractures through the magnetostrictive detection of acoustic emissions (AE). The present invention, in particular the detection of AE, also applies to NDT of other materials such as pipes, tubes, and plates.
2. Description of the Prior Art
The deterioration (corrosion and fracture) of individual wires which make up the main cables of structures such as suspension bridges, and the stays of cable stayed bridges, is a serious problem. Many of these bridges in the United States are well over 50 years old and the importance of addressing this problem has only increased in recent years. In order to repair and maintain these cables for bridge safety, a means for the nondestructive evaluation (NDE) of these cables for fractured wires and corrosion is urgently needed.
There currently are a number of NDE methods known in the art, whereby ultrasonic waves are used to detect the presence of breaks, fractures, corrosion, and the general deterioration of strands within a cable. Unfortunately, the ultrasonic methods described to date require some means for direct physical/acoustic contact to introduce the ultrasonic waves into the individual wires under study. Except for a few cases where the end of a cable is exposed and individual wires of the cable have sufficient exposure for transducer coupling, introducing ultrasonic waves into the individual wires in this manner is generally impractical.
The terminations, socketed areas, regions under the cable bands, and regions over the tower saddles are all generally inaccessible for existing NDE techniques such as DC magnetic field leakage and transverse impulse vibrational wave methods. These areas of cable are typically in direct contact with other structural members and, as such, cannot be readily analyzed using techniques that involve field leakage from the cable or vibrational analysis of the cable.
The stress forces associated with an ultrasonic wave traveling within a cable or metal strand change the magnetic induction of the ferromagnetic material due to the magnetostrictive effect. These changes in the magnetic induction within the cable or strand can be detected using a pick-up coil placed around or on the cable or strand.
U.S. Pat. No. 3,115,774, issued to Kolb, describes a magnetostrictive drill string logging device that incorporates a vibration sensor and takes advantage of the magnetostrictive properties of the metallic drill string. Unfortunately, the accuracy of the vibration sensor utilized in the Kolb patent greatly limits the ability of the patented device to help analyze or "log" the condition of the drill string within a drill hole. While the Kolb patent discloses the use of the magnetostrictive principle to generally analyze conditions along a ferromagnetic strand, it does not disclose an apparatus and method of sufficient refinement to allow a specific analysis of the corrosion, deterioration, or fractures that might be found in something such as a ferromagnetic steel cable.
An article published in 1982 in the book "Ultrasonic Testing" edited by J. Szilard, describes the application of magnetostrictive techniques to wire rope testing for fractured strands and corrosion. However, no description of an apparatus or a method for implementation of the concept is provided in this disclosure and, in general, the article simply describes or predicts the ability to use the physical magnetostrictive principle to detect defects in wire ropes or bridge cables. The Szilard article does disclose the use of a spiraling wave that is generated magnetostrictively to detect cracks within a single rod. Such spiraling waves, however, have a very limited range over which their analysis is practical. The technique described, therefore, does not lend itself to applications using long ropes or cables.
U.S. Pat. No. 4,979,125, issued to one of the inventors of the present application, describes a nondestructive means for evaluating wire ropes and cables by using the transverse impulse vibrational wave method. This method permits the detection of flaws by recognizing certain vibrational wave amplitude and distribution patterns resulting from striking the test cable or strand with a transverse force. Tension on a test strand or cable is calculated by measuring the propagation velocity of the vibrational waves through the test object. The distribution in both amplitude and time of the waves that result allows an analysis of the existence of flaws and variations in the tension across a length of rope that may not be accessible. This transverse vibrational wave method of analysis, however, is not appropriate for many areas where the vibration of the rope is effected by external components of the bridge structure or other external forces on the free movement of the wire strands.
It would, therefore, be advantageous to develop an NDE method useful for testing ferromagnetic wires, ropes, and wire strands and the like which would not require a direct access to the material under test. It would be advantageous if such a remote system could detect minor, as well as major rope flaws, stresses, and corrosions.
One method for monitoring the fracturing of wires in a steel cable or strand is by acoustic emission (AE) detection using a piezoelectric sensor. This requires a precise acoustic coupling with the strand through direct physical contact between the sensor and the strand. It also typically involves a certain amount of surface preparation such as the removal of paint and corrosion in the immediate area. The durability of such sensors, when maintained in direct contact with the cable strands, is very low. It is also quite complicated to install such piezoelectric sensors and careful analysis of the area of coupling is required to eliminate the effects of immediate structural discrepancies. These systems are also typically quite expensive, not only in apparatus costs, but in installation costs as well.
It is also known to use ultrasonic detectors to passively monitor acoustic emissions generated by progressing fractures within a cable or metallic strand. Typically, such applications utilize piezoelectric sensors that must be physically and acoustically coupled to the material under analysis. It is also apparent that frequency sensitivity limits the range and accuracy of such methods, even if the coupling requirements are met.